From ISO Standard 11898, for example, a Controller Area Network (CAN) as well as a “Time-Triggered CAN” (TTCAN) version of the CAN is known. The media access control method used in the CAN is based on a bit-wise arbitration. In a bit-wise arbitration, a plurality of user stations is simultaneously able to transmit data via the channel of the bus system, without thereby interfering with the data transmission. Furthermore, the user stations are able to ascertain the logical state (0 or 1) of the channel while transmitting a bit over the channel. If a value of the bit sent does not correspond to the ascertained logical state of the channel, the user station terminates the access to the channel. In CAN, the bit-wise arbitration is usually carried out in an arbitration field within a message that is to be transmitted via the channel.
Because of the bit-wise arbitration, a non-destructive transmission of the message over the channel is achieved. Because of this, good real time properties of the CAN come about, whereas media access control methods, in which the message sent by a user station is able to be destroyed during transmission via the channel, based on a collision with a further message sent by another station, have a clearly more unfavorable real time behavior since, based on the collision and the new transmission of the message required thereby, a delay in the data transmission comes about.
The protocols of the CAN are particularly suitable for transmitting short messages under real time conditions. If larger data blocks are to be transmitted via a CAN domain, the relatively low bit rate of the channel becomes a limiting factor. In order to assure the correct functioning of the bit-wise arbitration, during the arbitration, a minimum duration is must be maintained for the transmission of a bit, so that all the bus users have a uniform picture of the state of the bus (0 or 1) and equal priority access to the state of the bus, the minimum duration being a function of an extension of the bus system, the signal propagation speed on the channel, and intrinsic processing times in the interface modules of the bus users.
Therefore, increase of the bit rate would be problematic since this would reduce the duration of the individual bits. Since the signal propagation speed on the channel is essentially fixed, then, in case shorter bit lengths are to be achieved, a smaller extension of the bus system or lower intrinsic processing times are required. With respect to the intrinsic processing times, those times essentially correspond to that which is required until the respective bus level sets in. In this connection, one should note that, to set the dominant bus level, the sending station in each case, using a corresponding output stage, drives a current onto the bus which leads to the buildup of a corresponding voltage difference. In contrast, a recessive bus level sets in by the voltage difference between the two bus lines becoming reduced via terminating resistors or changing to a value deviating from the dominant bus level. This deviating value may be zero but may also be a voltage difference different from zero.
G. Cena and A. Valenzano, in “Overclocking of controller area networks” (Electronics Letters, vol. 35, No. 22 (1999), p. 1924) treat, from a theoretical point of view, the effects of overclocking the bus frequency in subsections of the message on the effectively achieved data rate, without, however, going into details of the methodology and the problem of the factors limiting the bus frequency, such as the signal propagation speed and intrinsic processing times in the interface modules of the bus users.
It may be seen in the cited documents that the related art does not supply satisfactory results from every point of view.